Project Summary Bacterial pneumonia remains a major health burden at the two ends of the age spectrum. It is the host response to these microbes that leads to both susceptibility and pathogenesis for this disease, emphasizing the need to better understand the host response which determines the occurrence and severity of pneumonia. Alveolar macrophages (AM) are long-lived cells capable of surviving infections and inflammation, but microbes can cause programmed necrosis of AM. Countering AM death decreases pneumonia severity, suggesting that this AM response is a lynchpin of lung defense. Dying AM are replaced by the local proliferation of yolk sac-derived AM plus the recruitment of bone marrow-derived monocytes which differentiate into AM. Distinctive biology of these 2 AM subsets is an important knowledge gap. Recovery from previous respiratory infections remodels lung immunity to make pneumonia less likely, involving several arms of adaptive and innate immunity that are beginning to be elucidated. Whether and how AM are altered in these previously infected and now protected lungs has not been examined. We hypothesize that recovery from previous respiratory infections remodels the pool of AM and increases resilience against microbe-induced programmed necrosis. We have collected preliminary data which support this central hypothesis and guide further investigation. We will examine our central hypothesis by pursuing the following specific aims: (1) To test the hypothesis that recovery from previous respiratory infections tilts the balance of yolk sac progenitor- vs. bone marrow hematopoiesis-derived AM in the lung, followed by distinctive responses of these 2 AM sub-types when those experienced lungs get infected. (2) To test the hypothesis that IFN-? renders alveolar macrophages more resilient. These aims will be approached using combinations of genetically engineered mice, blocking antibodies, recombinant cytokines, cell sorting, and RNAseq analyses. Elucidating what dictates macrophage resilience and finding ways to interfere with pathogen-induced macrophage programmed cell death will be steps towards new approaches for preserving macrophages to promote lung health and prevent pneumonia, to recognizing and potentially circumventing the natural loss of tissue resilience that accompanies aging, and to improving vaccine design to extend beyond antibody-mediated protection.